Membrane Potential Generated by Ion Adsorption

Abstract

It has been widely acknowledged that the Goldman-Hodgkin-Katz (GHK) equation fully explains membrane potential behavior. The fundamental facet of the GHK equation lies in its consideration of permeability of membrane to ions, when the membrane serves as a separator for separating two electrolytic solutions. The GHK equation describes that: variation of membrane permeability to ion in accordance with ion species results in the variation of the membrane potential. However, nonzero potential was observed even across the impermeable membrane (or separator) separating two electrolytic solutions. It gave rise to a question concerning the validity of the GHK equation for explaining the membrane potential generation. In this work, an alternative theory was proposed. It is the adsorption theory. The adsorption theory attributes the membrane potential generation to the ion adsorption onto the membrane (or separator) surface not to the ion passage through the membrane (or separator). The computationally obtained potential behavior based on the adsorption theory was in good agreement with the experimentally observed potential whether the membrane (or separator) was permeable to ions or not. It was strongly speculated that the membrane potential origin could lie primarily in the ion adsorption on the membrane (or separator) rather than the membrane permeability to ions. It might be necessary to reconsider the origin of membrane potential which has been so far believed explicable by the GHK equation.

Membrane potential generation without ion transport

“…Therefore, the potential in the system represented by Fig. 5 must be given by ϕ L -ϕ R according to Tamagawa and Morita's theory [14]. They successfully reproduced the experimentally observed potential behavior by the use of their theory.…”

“…Using such membranes, the potential across the impermeable membrane was investigated. Similar experiment was already carried out by Colacicco about 50 years ago [1,2,[14][15][16]. In the Colacicco's experiment, two ionic solutions were separated by an oil membrane.…”

“…The author of this paper already derived a formula representing the potential by taking the Langmuir isotherm [14]. Such an approach for deriving the formula of potential is the same as Drs.…”

“…So, the adsorption theory does not require the ion transport at all for the nonzero potential induction. Author of this paper, Tamagawa, and his collaborator, Morita, investigated the potential behavior across the impermeable membrane which separated two KCl solutions [14]. The potential behavior they observed was analyzed by their own theory.…”

“…4) [1,2,7,[9][10][11][12][13][14]. Therefore, it is important to experimentally study the behavior of potential across an impermeable membrane, too.…”